1. Field of the Invention
The present invention relates to a Shack-Hartmann sensor and a wavefront measuring method utilizing the same.
2. Description of the Related Art
A Shack-Hartmann sensor includes a micro lens array in which many micro condenser lenses are arranged in a lattice shape, and light receiving sensors such as CCDs, and light incident upon each micro lens condenses and forms a spot on a light receiving sensor. Herein, the “spot” has an intensity distribution of light that is condensed on the light receiving sensor by each micro lens. A center-of-gravity (“CoG”) position of the spot shifts due to a wavefront slope incident upon the micro lens array when the light is condensed, the wavefront slope is calculated based upon a CoG position shift amount of the spot, and a two-dimensional phase distribution can be calculated based upon the wavefront slope.
Each of Japanese Patent Laid-Open No. 9-15057 and Japanese Patent Domestic Publication No. 2002-535608 discloses a method for measuring a large wavefront aberration using a Shack-Hartmann sensor. In the large wavefront aberration, a local wavefront slope is steep and thus a positional shift amount of the spot increases. These methods enlarge a dynamic range by characterizing light that transmits one or more micro lenses and by correlating each micro lens with the spot. Japanese Patent Laid-Open No. 11-211562 discloses another illustrative wavefront measuring method.
When the large wavefront aberration is measured by the Shack-Hartmann sensor, due to the steep local wavefront slope and a large slope variation amount, a distance reduces between the CoG positions of the spots made by the light condensed by each micro lens. According to the above methods, even when each micro lens is correlated with the spot, as the distance between the CoG positions reduces the CoG position detecting precision lowers and finally the wavefront measuring precision lowers due to the crosstalk.